The present invention pertains to a process for controlling the valves of an internal combustion engine having at least two intake valves per cylinder. It also pertains to a system that permits the use of the process according to the present invention.
The present invention is applicable, in particular, to engines with 16 valves.
The reduction of the fuel combustion of vehicles is a major objective faced by the automobile industry.
One of the means of reducing the fuel consumption and/or significantly improving the pleasure of driving a vehicle equipped with an engine is to increase the torque of the engine at low engine speed in the range that is used predominantly by the driver. This improvement makes it possible, at equivalent performance, to increase the gear ratio of the vehicle and, by an effect induced by the modification of the operating point, to reduce the consumption of the vehicle.
The torque of an engine is linked directly with the amount of air that can be caused to enter the cylinders. The charge or the volumetric efficiency of an engine characterizes its capacity to admit air into its cylinders, assuming given conditions upstream (in terms of the pressure, temperature and humidity of the combustion air). For unsupercharged engines, the conditions upstream depend principally on the atmosphere.
The charge is defined as the ratio of the mass of air admitted into the cylinders during each cycle of the engine to the mass of the same volume of air (the displacement of the engine) under the conditions upstream.
The charge of an engine is not constant over the entire range of operation. Certain acoustic phenomena of the system formed by the air columns from the plenum of the intake distributor to the valves and the volume of air in the cylinders make it possible to improve this charge under certain operating conditions.
Under the resonance conditions of this system, it is possible to enclose in the cylinder a pressure that is higher than the atmospheric pressure and thus to take advantage of a natural supercharge. This phenomenon is called the Kadenacy effect and corresponds to the use of a Helmholtz tuning in analogy to a Helmholtz resonator or a mass-and-spring system. The high charge enables the engine to deliver a high torque under the operating conditions in which Helmholtz tuning occurs. The theoretical natural Helmholtz resonance frequency f is defined by the following formula:   f  =            c              2        ⁢        p              ⁢                  S        LV            in which
c is the velocity of sound in the medium contained in the intake circuit,
S is the mean cross section of the intake port (from the plenum of the intake distributor to the valves),
L is the length of an intake port, and
V is half the displacement plus the dead space of a cylinder.
Consequently, a single theoretical natural Helmholtz resonance frequency corresponds to a given geometry of the intake ports and a given displacement.
The theoretical Helmholtz tuning condition N is given by the formula:N=30*f* l/180in which:
f is the theoretical natural Helmholtz resonance frequency,
l is the width of the law of intake, which is the number of degrees of the crankshaft angle during which the intake valves are lifted by more than 1 mm.
It is also possible to increase the air charge of an engine taking advantage of a so-called quarter-wave acoustic tuning in the system formed by the primary tubes of the intake distributor. During the closing of the intake valves, the abrupt stopping of the movement being introduced from the air column present in the primary tube associated with this valve generates an overpressure wave which propagates toward the inlet of the primary tube. This wave is then reflected, changing its sign (depression wave), because the end of the primary tube is open over a considerable volume: the “plenum” of the distributor. When the depression wave arrives at the closed valve, it is reflected without changing its sign. It again reaches the open end of the primary tubes and is then reflected as an overpressure wave.
By adjusting the opening angle of the intake valve, this overpressure wave can be utilized to increase the rate of flow of the air being introduced at the beginning of the intake and thus to improve the charge.
The velocity of propagation of the waves in the primary ports being conventionally designated by C0, the propagation time of a wave from one end to the other end of a primary tube having a length l1 is: t=l1/C0. Considering the change in the sign of the wave at the time of its reflection in the plenum, the wave must perform an even number of back-and-forth movements in the same port to generate an overpressure at the valve. If the subsequent opening of the valve takes place at the end of a time that is a multiple of 4*l1/C0, the acoustic wave will have a beneficial effect on the opening of the intake valve.
In practice, the optimization of these acoustic effects by dimensioning the intake system and the adjustment of the laws of opening of the intake valves make it, in general, possible to benefit from these effects in a limited operating range. It follows from this that if the charge and consequently the torque are to be increased at low engine speeds, it is, in general, necessary to modify the dimensions and the adjustment, and this modification is manifested in a degradation of the engine performance at high engine speeds.
In case of conventional four-stroke gasoline engines, the opening and the closing of the intake valves are usually performed by a mechanical system, which leads to a fixed ratio between the lift of the valves and the angle of rotation of the engine regardless of the operating conditions or the charge of the engine. These engines are called engines with fixed camshaft adjustment.
However, variable timing systems are currently being developed, especially for four-stroke gasoline engines.
Thus, several types of prior-art systems make it possible to partially solve the above-mentioned problem:
the systems with variable acoustics comprising a mechanical device that makes it possible to vary the length of the intake ports and thus to vary the operating range that benefits from an acoustic tuning;
the camshaft phase shifting systems (VVT or VTC) which make it possible to vary the adjustment of the valve lift diagram in relation to the angle of rotation reference of the engine without modifying the lift diagram. The variation of the adjustment may be discrete or continuous; and
the systems with mechanical variable timing (“Valvetronic”), which make it possible to vary the moment of opening and the duration of opening identically for all intake valves.
These systems have the drawback of requiring arrangements or mechanical adjustments, which are not entirely satisfactory.
Consequently, the object of the present invention is to provide an easy-to-use system that makes it possible to notably improve the air charge of the cylinders of an internal combustion engine. It is particularly applicable in the gasoline-powered unsupercharged internal combustion engines equipped with variable timing systems that control the intake valves of each cylinder independently.